1. Field of the Invention
The invention relates to the manufacture, modification, and use of polymers that can interact with fillers.
2. Background of the Invention
Tire treads, power belts, and the like often are made from compositions that contain one or more elastomers and one or more reinforcing materials such as, for example, particulate carbon black and silica. For a general discussion of this topic, see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Safety and durability considerations mandate that tire treads provide both good traction and resistance to abrasion; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. The foregoing considerations are, to a great extent, competing and somewhat contradictory: a tire tread composition designed to improve tread traction on the road usually results in increased rolling resistance and vice versa.
Typically, filler(s), polymer(s), and additives are chosen so as to provide an acceptable balance of these properties. Ensuring that constituent reinforcing filler(s) are well dispersed throughout the elastomeric material(s) in such compositions both enhances processability and acts to improve physical properties such as, e.g., compound Mooney viscosity, elastic modulus, tan δ, and the like. Resulting articles made from such compositions can exhibit desirable properties such as reduced hysteresis, reduced rolling resistance, and good traction on wet pavement, snow and ice.
Dispersion of fillers can be improved by increasing their interaction with the elastomer(s) in which they are to be dispersed. Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemical modifications to terminal ends of the polymers with, e.g., amines, tin compounds, and the like.
Because elastomers used in such compositions often are made via anionic polymerization techniques, attachment of certain functional groups, particularly amines, is difficult. Living polymers are terminated by active hydrogen atoms such as are present in, e.g., hydroxyl groups, thiol groups, and particularly primary and secondary amine groups. This undesired termination can be avoided through use of reaction schemes that allow for attachment of a non-amine N-containing compound followed by conversion to an amine, i.e., indirect attachment schemes.
Rubber articles at times can suffer from the growth of cracks or structural defects due to fatigue of the rubber, often referred to as fatigue crack growth (hereinafter “FCG”). Formation and growth of cracks can play a large role in determining the useful service lifespan of such rubber articles. Efforts to inhibit or reduce FCG tend to focus on provision of elastomers with low crosslink density (which reduces tear energy) and/or inclusion of stiff components in or with the elastomer (which form hard domains that can block propagation of fatigue-induced defects such as cracks).
Additionally, lower molecular weight polymers can be used as processing aids; specifically, they can assist in lowering the mixing viscosity of rubber compounds. Unlike processing oils, lower molecular weight polymers typically do not exude from the rubber matrix and, instead, are incorporated into the vulcanizate. However, lower molecular weight polymers can complicate processing of rubber compounds because of their poor cold flow properties. The use of a coupling agent can alleviate this cold flow deficiency, although one type (such as, e.g., SnCl4) tends break down completely during processing while another (such as, e.g., SiCl4) does not break down at all.
Continued hysteresis reduction, provision of a direct mechanism for attaching amine functionality to a living polymer, inhibition of FCG, and controlled breakdown of coupled low molecular weight polymers all remain highly desirable.